FIELD OF THE INVENTION
The present invention relates to a disc wheel for a passenger car.
Conventionally, the plate-like disc in a discs used wheel for motor vehicles such as passenger cars, motor trucks and the like are press formed. In such press formation, it is difficult to set the distribution of the thickness of the disc to a preferred level, and a description of the problems resulting follows.
Generally, in order to emit a breaking heat, the disc wheel for a motor vehicle is formed with a ventilation hole 103 in a portion outside a hat portion 102 of a disc 101, as shown in FIGS. 25A and 25B. Accordingly, this ventilation hole portion 104 generally weakens in a rotational bending fatigue test and a crack 105 is generated in the ventilation hole portion 104, as shown in FIGS. 26A, 26B and 26C. In order to strengthen this, it is necessary to increase the thickness of the portion 104. Further, in some cross sectional shapes, a bolt hole 10 may weaken the hub mounting portion 106 and generate a crack 105.
Accordingly, in the structure formed by a press molding as mentioned above, the ventilation hole portion 104 or the hub mounting portion 106 can only have adequate strength by being as thick as the whole of the disc, and must be at least as thick as the weakest portion. Accordingly, unless the strength of the ventilation hole portion 104 or the hub mounting portion 106 can be improved, a reduction of weight is limited.
Furthermore, when the thickness of the whole of the disc is increased, the stress in the welding portion between the disc and a rim generates a crack 105' in the rim drop portion of a welding portion WO, as shown in FIG. 26A.
Moreover, in order to prevent interference with the brake portion 400 as shown in FIGS. 4 and 10 and improve the design, the shape of the disc is frequently limited. Accordingly, an optimum shape is impossible to achieve and the stress balance deteriorates, so that in many cases, the fatigue life cannot be improved without increasing the thickness. As mentioned above, when partially increasing thickness, it is necessary to increase the thickness of the whole disc and the amount of weight that can be reduced is limited
Furthermore, in a double mounting type disc wheel used in a motor truck and a motor bus, the thickness of the joint welding portion 203 in the disc 202 with respect to a rim 201 is less than the thickness of a hub mounting portion 204, and the thickness of the hat portion 205 is less than the thickness of the joint welding portion 203, as shown in FIG. 27, which produces a disc wheel having uneven thickness. This type of disc wheel is, for example, disclosed in Japanese Patent Unexamined Publication No. 7-257101. This structure aims to prevent stress on the joint welding portion 203 in the disc, improve the fatigue strength and to reduce weight. However, if a disc wheel with uneven thickness is used in the hat portion, ventilation holes are formed in the thin hat portion 205, so that the crack mentioned above is generated in the hat portion 205
Also, as shown in FIG. 27, the thickness of the flange portion corresponding to the joint welding portion 203 in the disc 202 is generally greater than the thickness of the hat portion 205. Although, the flange portion 203 may have a low stress ratio and a reduced thickness but the rim can be made thinner than the disc, and a significant difference in thickness between the rim and the disc can cause stress that easily produces a crack in the rim side of the welding portion W.sub.O with respect to the wheel strength.
Accordingly, as mentioned above, when the flange portion in the disc 202 is thick, it is necessary to make the rim thick in order to secure fatigue strength and reduce the difference in thickness, so that it increases the weight of the wheel.
Furthermore, although a disc wheel with uneven thickness in a passenger car shown in FIG. 28, and disclosed in Japanese Patent Unexamined Publication No. 1-104423, contains a method of applying an excess metal 303 to a hat R portion 302 of a disc 301 so as to increase the thickness, the thickness of the ventilation hole portion 304 is not increased in the disc and the same crack mentioned above is easily generated in the ventilation hole portion 304.